Process for making flexible foams

ABSTRACT

Process for preparing a flexible foam by reacting a prepolymer having an NCO value of 3-15% by weight, which is the reaction product obtained by reacting an excessive amount of a polyisocyanate with a polyether polyol or a mixture of such polyols, said polyol or mixture having an average nominal hydroxyl functionality of from 2 to 6, an average hydroxyl equivalent weight of from 500 to 5000 and an oxyethylene content of at least 50% by weight, with water, the amount of water being 15-500 parts by weight per 100 parts by weight of prepolymer, at the start of the reaction the temperature of the prepolymer being 10°-50° C. and the temperature of the water being 10°-50° C. higher than the temperature of the prepolymer and a process for preparing a flexible foam by reacting the above prepolymer with water in the presence of a superabsorbent polymer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/463,588 filed Jun. 5, 1995 and U.S. patent application Ser.No. 08/478,078, filed Jun. 7, 1995, both now abandoned.

The manufacture of polyurethane flexible foams by reacting organicpolyisocyanates such as toluene diisocyanates (TDI) or diphenyl methanediisocyanates (MDI) with polyether polyols in conjunction with a foamingagent is well established. The polyethers are usually polyoxypropylenepolyols derived from propylene oxide or poly(oxypropylene-oxyethylene)polyols derived from various combinations of propylene and ethyleneoxides. Ethylene oxide tipped polyoxypropylene polyols wherein theoxyethylene groups constitute a minor proportion of the totaloxyalkylene residues are particularly important because of theirenhanced reactivity towards isocyanates.

Polyols having higher oxyethylene contents, for example 50% or more on aweight basis, are often employed as minor additives to ensure that thefoams have an open-cell structure. The use of these polyethers at veryhigh concentrations in conjunction with the usual isocyanates is not asusual because then, instead of having a cell-opening effect, they resultin closed cell foam.

In co-pending application PCT/EP94/01659 it has been found that flexiblefoam having valuable properties can be successfully made fromformulations containing high concentrations of polyols having highoxyethylene contents if substantially pure 4,4'-MDI or a derivativethereof is employed as the polyisocyanate, water being used as theblowing agent. The preparation of hydrophilic flexible foams has furtherbeen described in U.S. Pat. No. 4,137,200 and U.S. Pat. No. 4,828,542.

Surprisingly it has now been found that hydrophilic foams may beobtained when the prepolymer, made from a polyisocyanate and a polyolhaving a high oxyethylene content, and the water are used at differenttemperatures.

Thus according to the invention, there is provided a process for thepreparation of flexible foams by reacting a prepolymer having an NCOvalue of 3-15% by weight, which is the reaction product obtained byreacting an excessive amount of a polyisocyanate with a polyether polyolor a mixture of such polyols, said polyol or mixture having an averagenominal hydroxyl functionality of from 2 to 6 and preferably of from 2to 4, an average hydroxyl equivalent weight of from 500 to 5000 andpreferably from 1000 to 5000 and an oxyethylene content of at least 50%by weight, with water, the amount of water being 15-500 parts by weightper 100 parts by weight of prepolymer, characterised in that at thestart of the reaction the temperature of the prepolymer is 10°-50° C.,preferably 15-30° C. and most preferably room temperature and thetemperature of the water is 10°-50° C., preferably 20°-45° C. higherthan the temperature of the prepolymer. The temperature of the water is25°-90° C., preferably 40°-70° C., most preferably 55°-65° C.

A preferred embodiement of the invention is a method for the preparationof flexible polyurethane foams by reacting a prepolymer, having an NCOvalue of 3-10% by weight which is the reaction product obtained byreacting an excessive amount of a polyisocyanate containing at least 65,preferably at least 90, and more preferably at least 95% by weight of4,4'-diphenyl methane diisocyanate or a variant thereof with a polyetherpolyol or a mixture of said polyols, said polyol or mixture having anaverage nominal hydroxyl functionality of from 2.5 to 3.5, an averagehydroxyl equivalent weight of from 1000 to 3000, and an oxyethylenecontent of from 50 to 85% by weight, with water, the amount of waterbeing 30-300 parts by weight per 100 parts by weight of prepolymer,characterised in that at the start of the reaction the temperature ofthe prepolymer is 10°-50° C., preferably 15°-30° C. and most preferablyroom temperature and the temperature of the water is 25°-90° C.,preferably 40°-70° C. and most preferably 55°-65° C. and the temperatureof the water is 10°-50° C., preferably 20°-45° C. higher than thetemperature of the prepolymer.

Surprisingly it has been found that good quality hydrophilic flexiblefoams can be obtained having a low density and hardness while thedensity and the hardness of the foam become less or even hardlydependent upon the amount of water used than in case the prepolymer andthe water are reacted while having both the same or a similartemperature at the start of the reaction. For the sake of conveniencethe word average in the present application is not further specified butrefers to number average unless explicitly used otherwise.

Polyisocyanates used for preparing the prepolymer may be selected fromaliphatic, cycloaliphatic and araliphatic polyisocyanates, especiallydiisocyanates, like hexamethylene diisocyanate, isophorone diisocyanate,cyclohexane-1,4-diisocyanate, 4,4'-dicyclohexylmethane diisocyanate andm- and p- tetramethylxylylene diisocyanate, and in particular aromaticpolyisocyanates like toluene diisocyanates (TDI), phenylenediisocyanates and most preferably methylene diphenylene diisocyanates(MDI) and its homologues having an isocyanate functionality of more thantwo, like crude MDI and polymeric MDI.

Preferred polyisocyanates are methylene diphenylene diisocyanatesselected from pure 4,4'-MDI, isomeric mixtures of 4,4'-MDI and 2,4'-MDIand less than 10% by weight of 2,2'-MDI, and modified variants thereofcontaining carbodiimide, uretonimine, isocyanurate, urethane,allophanate, urea or biuret groups, like uretonimine and/or carbodiimidemodified MDI having an NCO content of at least 25% by weight andurethane modified MDI obtained by reacting excess MDI and a lowmolecular weight polyol (MW up to 1000) and having an NCO content of atleast 25% by weight. Mixtures of the isocyanates mentioned above may beused if desired. The polyisocyanate may contain dispersed urea particlesand/or urethane particles prepared in a conventional way, e.g. by addinga minor amount of an isophorone diamine to the polyisocyanate.

The most preferred polyisocyanate used in preparing the prepolymer is apolyisocyanate containing at least 65%, preferably at least 90% and morepreferably at least 95% by weight of 4,4'-diphenyl methane diisocyanateor a variant thereof. It may consist essentially of pure 4,4'-diphenylmethane diisocyanate or mixtures of that diisocyanate with one or moreother organic polyisocyanates, especially other diphenyl methanediisocyanate isomers, for example the 2,4'-isomer optionally inconjunction with the 2,2'-isomer. The most preferred polyisocyanate mayalso be an MDI variant derived from a polyisocyanate compositioncontaining at least 65% by weight of 4,4'-diphenylmethane diisocyanate.MDI variants are well known in the art and, for use in accordance withthe invention, particularly include liquid products obtained byintroducing uretonimine and/or carbodiimide groups into saidpolyisocyanates, such a carbodiimide and/or uretonimine modifiedpolyisocyanate preferably having an NCO value of at least 25% by weight,and/or by reacting such a polyisocyanate with one or more polyols havinga hydroxyl functionality of 2-6 and a molecular weight of 62-1000 so asto obtain a modified polyisocyanate, preferably having an NCO value ofat least 25% by weight.

The polyether polyol or mixture of polyether polyols used in preparingthe prepolymer preferably has an average nominal hydroxyl functionalityof 2-4 and most preferably of 2.5-3.5 and an average hydroxyl equivalentweight of 1000-3000 and an oxyethylene content of from 50-85% by weight.

Polyether polyols include products obtained by the polymerisation ofethylene oxide optionally together with another cyclic oxide liketetrahydrofuran and--preferably--propylene oxide in the presence, wherenecessary, of polyfunctional initiators. Suitable initiator compoundscontain a plurality of active hydrogen atoms and include water,butanediol, ethylene glycol, propylene glycol, diethylene glycol,triethylene glycol, dipropylene glycol, ethanolamine, diethanolamine,triethanolamine, toluene diamine, diethyl toluene diamine, phenyldiamine, diphenylmethane diamine, ethylene diamine, cyclohexane diamine,cyclohexane dimethanol, resorcinol, bisphenol A, glycerol,trimethylolpropane, 1,2,6-hexanetriol, pentaerythritol and sorbitol.Mixtures of initiators may be used.

If another cyclic oxide is used the polyol may be obtained by thesimultaneous or sequential addition of ethylene oxide and the othercyclic oxide as fully described in the prior art.

In order to obtain the preferred polyol having an average nominalhydroxyl functionality of 2.5 to 3.5 a polyol having a nominal hydroxylfunctionality of 3 may be used or a mixture of polyols having an averagenominal hydroxyl functionality of 2-6 provided the mixture is in theabove 2.5-3.5 functionality range.

In general polyol mixtures may be used provided they have the requiredfunctionality, equivalent weight and oxyethylene content as describedabove.

The term "average nominal hydroxyl functionality" is used herein toindicate the average functionality (number of hydroxyl groups permolecule) of the polyol composition on the assumption that the averagefunctionality of the polyoxyalkylene polyols present therein isidentical with the average functionality (number of active hydrogenatoms per molecule) of the initiator(s) used in their preparationalthough in practice it will often be somewhat less because of someterminal unsaturation.

If desired, the polyether polyol or the mixture of polyols may containdispersed polymer particles. Such polymer-modified polyols have beenfully described in the prior art and include products obtained by the insitu polymerisation of one or more vinyl monomers, for exampleacrylonitrile and styrene, in polyoxyalkylene polyols or by the in situreaction between a polyisocyanate and an amino- or hydroxy-functionalcompound, for example triethanol-amine, in the polyoxyalkylene polyol.

The prepolymer is prepared conventionally by reacting the polyisocyanateand the polyol at relative amounts so as to obtain an NCO value of 3-15%by weight, preferably of 3-10% by weight at a temperature preferablybetween 40° and 90° C. The prepolymers so prepared are liquid at ambientconditions. To the prepolymer so prepared low amounts (up to 30% byweight) of further polyisocyanate and in particular MIDI may be added ifdesired. In order to improve the stability of the prepolymer an organicacid or Lewis acid may be added.

The prepolymer preferably has a viscosity of at most 10.000 mPa.s at 25°C.

In preparing the prepolymer it should be avoided that the isocyanatefunctionality of the polyisocyanate and the average nominal hydroxylfunctionality of the polyol or mixture of polyols are both 2.0. If oneof these functionalities is 2.0 the other one preferably is at least2.2.

The prepolymer is reacted with water, the amount of water being 15-500,preferably 30-300, most preferably 40-250 parts by weight per 100 partsby weight of prepolymer.

The foam-forming reaction mixture may contain one or more of theadditives used in preparing flexible foams. Such additives includecatalysts, for example tertiary amines and tin compounds, surface-activeagents and foam stabilisers, for example siloxane-oxyalkylene copolymersand polyoxyethylene/polyoxypropylene copolymers and polyoxyethylenepolymers, chain extenders, for example low molecular weight diols ordiamines, cross-linking agents, for example triethanolamine, glyceroland trimethylolpropane, flame retardants, organic and inorganic fillers,pigments, agents for suppressing the so-called boiling-foam effect likepolydimethylsiloxanes, internal mould release agents, anticeptics,biocides and medicaments. However, valuable flexible foams may beobtained without any of these additives. Preferably no additives areapplied except up to 10 parts and preferably up to 5 parts by weight ofthe aforementioned polyoxyethylene/polyoxypropylene copolymers andpolyoxyethylene polymers per 100 parts by weight of prepolymer. If used,such additives preferably are pre-mixed with the water. As to the use ofthese (co)polymers it was surprisingly found that foams, which exhibitvery good wicking properties and are able to absorb and retain water inan amount of several times the weight of the foam and/or which havereticulated cells, could be made when the prepolymer and the water arereacted in the presence of 0.01-10 parts by weight, per 100 parts byweight of prepolymer, of a polyol having an average molecular weight of500-10000 and an average nominal hydroxyl functionality of 2-6, thispolyol being a polyoxyethylene polymer or a polyoxyethylenepolyoxypropylene block copolymer having an oxyethylene content of atleast 30% by weight. This polyol preferably is used in an amount of0.05-3 parts by weight per 100 parts by weight of prepolymer. Thesepolyols are known in the art and commercially available. Examples areSynperonic™ PE L44, L64, F68, P75, P84, P85 and F87, all available fromImperial Chemical Industries PLC.

When these polyols are used the wicking properties are especiallyobtained when polyols are used having an oxyethylene content of 35-70and more in particular 40-70% by weight; preferably the average nominalhydroxyl functionality of such polyols is 2. When these polyols are usedthe reticulated properties are especially obtained when polyols are usedhaving an oxyethylene content of 70-100 and optimally 100% by weight; itis preferred to use at least 40 parts by weight of water per 100 partsby weight of prepolymer for preparing such reticulated foams.

Before the prepolymer and the water are reacted in the presence of thispolyol, the water and this polyol preferably are premixed.

Further the process according to the present invention using water at atemperature higher than the prepolymer may be conducted in the presenceof superabsorbent polymers as will be explained hereinafter in moredetail. The types and amounts of superabsorbent polymers and the waythey are used are described below.

This reaction system is used for making hydrophilic flexible foams,having good properties, in a very simple way. The preferred prepolymershave a low viscosity which improves ease of handling and processing whenmaking flexible foams which have a desirable colour (white), have opencells or are easy crushable and can have compression set values (ASTM D3574-77, Test D, dry 50%) below 20% in particular when no surfactant isused. The purity and simplicity of the chemicals used for making theprepolymers ensures that flexible foams made thereof have a minimum ofleachable substances which make these foams especially useful in areaswhere contact with a human body is required like in medical and hygienicapplications.

The foams may be produced in the form of slabstock, mouldings and thelike and may be used for vibration damping, diapers, sponges, wounddressings, tampons, cosmetic pads, drug release products, plant growthmedia, absorbent in food trays and the like.

Further the present invention is concerned with a process for preparinga hydrophilic flexible foam by reacting the prepolymer as defined beforewith 15-500 parts by weight of water per 100 parts by weight ofprepolymer in the presence of a superabsorbent polymer.

Superabsorbent polymers (SAP) are widely known as such. SAP orwater-absorbent polymers or hydrogels are water-insoluble hydrophilicpolymers, able to swell and absorb amounts of water, saline solutions,physiological fluids or body fluids as high as 10-100 times their ownweight. They consist of polyelectrolytes or other highly hydrophilicpolymeric matrices, usually bearing crosslinking sites along themacromolecular chains in order to avoid dissolution. They may be naturalSAPs, like guar gum, other natural gums and starches and, preferably,synthetic SAPs which include polymers based on acrylic or methacrylicacids, esters, nitriles, amides and salts therof, polysaccharides,maleic anhydride polymers, poly(vinyl) alcohol,poly(N-vinyl-pyrrolidone) and diallyl dialkyl quaternary ammonium salts.For an overview of SAP we refer to a review article "Water-AbsorbentPolymers: A Patent Survey" of Riccardo PO in J.M.S--Rev. Macromol. Chem.Phys., C34 (4), 607-662 (1994). The superabsorbent polymers disclosed inthis article may be used in the present invention.

SAPs based on acrylic or methacrylic monomers are polymers made by freeradical polymerization of acrylic or methacrylic acids, esters,nitriles, amides and/or salts thereof optionally together with otherunsaturated monomers like maleic, fumaric or itaconic derivatives, vinylsubstituted sulfonic or ammonium salts, olefinic and styrenic monomers,hydroalkyl or alkyl acrylates and methacrylates, unsaturated sulfonicacid salts, acrylamidoalkyl sulfonic salts, vinyl sulfonate, styrenesulfonate, vinylbenzyl sulfonate, N,N'-methylenebisacrylamide,dialkylaminoalkyl acrylate and methacrylate, carbonyl containingheterocyclic N-vinyl monomers like N-vinyl-2-pyrrolidone,N-vinyl-2-caprolactam and N-vinyl-2-morpholinone. The polymers may beprepared by processes known in the art using if desired initiators,cross-linkers and surfactants as known in the art; see e.g. PO, pages610-632. Cross-linking may be carried out by free radicalcopolymerization of small amounts of polyvinylic comonomers or byreaction of the pendent carboxylate or carboxylic groups of the polymerwith a polyepoxide, a haloepoxide and/or a polyol.

Polysaccharide based superabsorbent polymers may be selected from e.g.starch graft copolymers and modified cellulose polymers. Such SAPs areobtained by grafting an unsaturated monomer like acrylonitrile, acrylicacid or acrylic amide onto a polysaccharide like starch or cellulose,optionally followed by saponification. Such polysaccharide basedsuperabsorbent polymers are known in the art and may be prepared byprocesses known in the art; see PO, pages 632-638.

SAPs based on maleic anhydride polymers are made by reacting maleicanhydride and hydrophobic comonomers like an olefin or vinylether byprocesses known in the art; see PO, pages 638-642.

Further SAPs which may be used are polymers prepared by polymerizingdiallyl dialkyl quaternary ammonium salts in the presence of apolyfunctional divinyl compound and/or a crosslinker like triallylmethylammonium chloride; polyalkylene oxides like polyethyleneoxidewhich have been cross-linked with e.g. formaldehyde and glutaraldehydein the presence of sulphuric acid; poly(N-vinyl pyrrolidone) andpoly(N-methyl, N-vinyl acetamide) which have been cross-linked by meansof e.g. divinylbenzene, diacrylates or diethylene glycol divinylether.Such SAPs and the way they are made are known in the art; see PO, pages642-647.

Preferred SAPs are selected from super absorbent polymers based onacrylic or methacrylic acids, esters, nitriles, amides and/or saltsthereof; polysaccharide based superabsorbent polymers and superabsorbentpolymers based on maleic anhydride.

SAP may be used in polyurethane foams. There exist three ways toincorporate SAP in polyurethane foam:

1. The ingredients for making the SAP and the ingredients for making thepolyurethane foam are combined and the SAP and the polyurethane foam areallowed to be formed at the same time; see e.g. U.S. Pat. No. 4,731,391and EP-163150. The result is an interpenetrating network. Disadvantageof this approach is that due to the presence of a relatively largenumber of reacting chemicals the process is cumbersome and difficult tocontrol and that often monomers for the SAP preparation are used whichare dangerous and toxic; this requires the flexible foam producer totake additional measures to protect the safety and health of itsemployees and the environment.

2. The SAP is incorporated in the polyurethane foam by impregnationusing a liquid as a carrier for the SAP; see EP-41934. A disadvantage ofthis approach is that the preparation of the SAP containing foamrequires a number of additional process steps which makes the processeconomically less attractive; further the cell size of the flexible foamneeds to be bigger than the size of the swollen SAP particles whichmeans a serious restriction as to the size of the SAP particles.

3. The SAP particles are mixed with the ingredients used for making theflexible foam.

EP-453286 discloses a superabsorbent foam material based on polyurethanefoam and containing superabsorbent material. The superabsorbentmaterial, which may be selected from the known ones, is mixed withconventional polyurethane formulations, which are then used to make apolyurethane foam. Such formulations contain a polyol, a catalyst, apolyisocyanate and a low amount of water.

U.S. Pat. No. 5,336,695 describes hydrophilic foams based on apolyurethane gel obtainable from a polyol, a diisocyanate, asuperabsorbent, a catalyst and a low amount of water.

U.S. Pat. No. 4,201,846 discloses the use of a fiber made from a polymerof polyvinyl alcohol in a hydrophilic foam in order to reduce theswelling of the foam. The polyurethanes are made by reacting aprepolymer and water in the presence of the fiber and show improvedtensile and tear properties and a reduced volume increase due to waterabsorption.

U.S. Pat. No. 3,900,030 discloses hydrophilic foams containing a finelydivided, water-swellable polymer for use in tampons. The amount ofpolymer is critical in order to avoid leakage. The foams are made byreacting a mixture of the polymer and a polyol with a diisocyanate inthe presence of a catalyst and a low amount of water.

U.S. Pat. No. 4,603,076 discloses the preparation of a hydrophilic foamby blowing an MDI-based prepolymer with a non-aqueous blowing agent anda polyoxyethylene polyol in the presence of a hydrophilic compound and acatalyst. The prepolymer is based on a mixture of MDI and polymeric MDI.

U.S. Pat. No. 4,985,467 discloses the preparation of hydrophilic foamsby reacting a polyisocyanate, a polyol and water in the presence of asuperabsorbent material followed by thermal reticulation.

EP-547765 and WO94/29361 disclose the preparation of flexible foamsusing a prepolymer, made from 4,4'-MDI and a polyether polyol having anoxyethylene content of 50-85% by weight, and water.

Surprisingly, we have found that hydrophilic flexible foams can beprepared using such superabsorbent polymers as discussed above byreacting a prepolymer as described above with high amounts of water inthe presence of such a superabsorbent polymer (SAP). Such foams can bemade without thermal reticulation.

The SAP may be premixed with the prepolymer, which makes the processvery simple for the foam producer: only water needs to be added.

The foams are also attractive from an environmental point of view. Apartfrom the prepolymer, the SAP and the water no further additives areneeded, in particular no catalysts.

The foams have very desirable properties: they exhibit limitedskrinkage, have open-cells, are stable, do not show scorching and havevery good water-absorbtion and water-retention characteristics, verygood wicking properties and mechanical properties like tear strength(dry and wet) and elongation. Further the foams have a soft feel,provided no reticulated foams are made using the polymers and copolymersdescribed before.

Consequently the present invention is further concerned with a processfor preparing a flexible foam by reacting a prepolymer having an NCOvalue of 3-15% by weight, which is the reaction product obtained byreacting an excessive amount of a polyisocyanate with a polyether polyolor a mixture of such polyols, said polyol or mixture having an averagenominal hydroxyl functionality of from 2 to 6 and preferably of from 2to 4, an average hydroxyl equivalent weight of from 500 to 5000 andpreferably of from 1000 to 5000 and an oxyethylene content of at least50% by weight, with water, the amount of water being 15-500 parts byweight per 100 parts by weight of prepolymer, characterised in that thereaction of the prepolymer and the water is conducted in the presence ofa superabsorbent polymer. Still further the present invention isconcerned with a compositon comprising a prepolymer having an NCO valueof 3-15% by weight, which is the reaction product obtained by reactingan excessive amount of a polyisocyanate with a polyether polyol or amixture of such polyols, said polyol or mixture having an averagenominal hydroxyl functionality of from 2 to 6, an average hydroxylequivalent weight of from 1000 to 5000 and an oxyethylene content of atleast 50% by weight and a superabsorbent polymer.

The amount of superabsorbent polymer used in general will be 1 to 100parts by weight (pbw) per 100 parts by weight of prepolymer and morepreferably 5 to 80 pbw and most preferably 10 to 70 pbw.

The prepolymers which may be used in this process according to thepresent invention and the way foams are made thereof are the same asthose described above. The amounts of water used preferably are asdescribed before.

In particular the foams preferably are made in the absence of theadditives, especially catalysts, mentioned before with the exception ofthe polyoxyethylene polymers and polyoxyethylene/polyoxypropylenecopolymers mentioned before.

When foams having particularly good wicking properties are desired orwhen foams showing reticulation are desired the same measures may betaken as described before.

The SAPs may be selected from those described in the article of PO asdiscussed before. More in particular they may be selected fromcross-linked polyacrylates and polyacrylamides and the salts thereof.Such SAPs are commercially available; e.g. SANWET™ IM 3900G, IM 3746/1and E394-95 from Hoechst/Cassella. Further SAPs may be selected fromstarch or cellulose grafted SAPs, using e.g. acrylonitrile, acrylic acidor acrylic amide as the unsaturated monomer. Such SAPs are alsocommercially available; e.g. SANWET IM7000 from Hoechst/Cassella.

Different SAPs may be used in combination. The SAPs may be mixed withthe prepolymer and the water at the moment this prepolymer and water aremixed or the SAPs are premixed with the prepolymer. Preferably the SAPsare not premixed with the water. The mixing may be conducted by means ofhand-mixing or normal machine mixing or under high shear mixingconditions.

Further the invention is concerned with absorbent articles, likediapers, sponges, wound dressings and tampons, comprising acatalyst-free, hydrophilic polyurethane flexible foam containingsuper-absorbent polymers. Preferably the foam is based on diphenylmethane diisocyanate. The invention is further illustrated by means ofthe following Examples.

EXAMPLE 1

Polyol 1 is a polyether (triol-initiated) having random oxyethylene andoxypropylene residues with a 77% oxyethylene content and a molecularweight of about 4000. A prepolymer was prepared by reacting 70 parts byweight of polyol 1 and 30 parts by weight of 4,4'-MDI. From thisprepolymer a flexible foam was prepared by reacting it with varyingamounts of water containing 0.4% by weight of Pluronic PE 6200 (an EO/POsurfactant from BASF-Pluronic is a trademark). The prepolymer had an NCOvalue of 7.85% by weight and a viscosity of 6000 mPa.s at 25° C. Thetemperature of the prepolymer was at room temperature (22° C.) prior toreaction. The amount and temperature of the water prior to reaction andthe density and the hardness of the foams obtained is given in Table 1below. Density and hardness were measured after drying the foam in anoven at 60° C. till no weight reduction was observed.

                  TABLE 1                                                         ______________________________________                                        Experiment 1*     2*     3*  4*   5   6    7    8                             ______________________________________                                        Amount of water,                                                                         30     110    30  110  30  110  30   110                           pbw per 100 pbw                                                               of prepolymer                                                                 Temperature of                                                                           10     10     25  25   45  45   65   65                            water, °C.                                                             Core Density,                                                                            75     170    72  144  68  91   64   74                            kg/m.sup.3                                                                    Hardness, CLD                                                                            9.1    17.1   7   14.5 5   6.4  3.6  4.3                           40% (kPa)                                                                     ISO 3386                                                                      ______________________________________                                         *comparative experiments                                                 

EXAMPLE 2

The following Synperonic surfactants were used.

                  TABLE 2                                                         ______________________________________                                                     EO content,                                                                   % by weight                                                                            molecular weight                                        ______________________________________                                        Synperonic PE                                                                             L 43   30         1900                                                        L 44   40         2200                                                        L 64   40         2900                                                        P 84   40         4200                                                        P 85   50         4650                                                        F 87   70         7700                                                        F 38   80         4800                                            ______________________________________                                    

From 100 parts by weight of the prepolymer used in example 1 a flexiblefoam was prepared by reacting it with 70 parts by weight of watercomprising 0.56 parts by weight of Synperonic polyol. The temperature ofthe prepolymer before the reaction was 22° C. The temperature of thewater before the reaction, the type of Synperonic used and wickingresults are given in Table 3.

                  TABLE 3                                                         ______________________________________                                                Synperonic  Wicking test                                                                             Water temperature                              Experiment                                                                            PE type     (seconds)  (°C.)                                   ______________________________________                                        1       L 43        53         45                                             2       L 44        11         45                                             3       L 64        8          45                                             4       P 84        2          45                                             5       P 85        1          45                                             6       F 87        5          45                                             7       F 38        130        45                                              8*     L 64        238        20                                              9*     P 84        254        20                                             10*     P 85        267        20                                             ______________________________________                                         *comparative examples                                                    

The foams obtained in experiments 8-10 have predominantly closed cells.

Wicking test: a dry foam sample of size 9×9×1 cm is put on a watersurface (one of the two large sides of the sample is put on the water)and the time is recorded until the upper face of the sample is visuallywet completely.

Foams were made from the above prepolymer (100 pbw, at 22° C.), waterand surfactant (0.8% by weight in the water). In Table 4 the amount (inpbw per 100 pbw of prepolymer) and the temperature of the water and thetype of surfactant has been given together with an indication whetherthe foam obtained was a reticulated foam or not.

                  TABLE 4                                                         ______________________________________                                                                  Water                                                                 Water   temperature,                                                                           reticulated                                Experiment                                                                            Surfactant                                                                              amount  °C.                                                                             foam                                       ______________________________________                                        11*     G 26      70      25       no                                         12      G 26      70      60       yes, completely                            13      F 68      70      80       yes, completely                            14*     F 87      110     25       no                                         15      F 87      110     45       yes, majority                                                                 of the cells                               16*     P 75      110     25       no                                         17      P 75      110     45       yes, minority                                                                 of the cells                               ______________________________________                                         *comparative experiments                                                 

G26 is a polyoxyethylene triol of molecular weight of 1200.

F68 is Synperonic PE F68: EO content 80% by weight and MW=8350.

P75 is Synperonic PE P75: EO content 50% by weight and MW=4150.

EXAMPLE 3

100 pbw of the prepolymer of example 1 was reacted with 70 pbw of watercontaining 0.8% by weight of Synperonic L64. The temperature of theprepolymer and the water before the reaction was room temperature (22°C.) and 45° C. respectively. Before the water and the prepolymer werecombined and mixed SAP was added to the prepolymer and mixed. The typeand amount (pbw per 100 pbw of prepolymer) of SAP used is given in Table5, experiment 1-10 together with the results: the core density (kg/m³)of the foam obtained was measured according to ASTM 3574/A; the maximalamount of 0.9% NaCl aqueous solution (physiological salt solution) whichcould be absorbed by the foam was determined (as grams of solution perdm³ of foam); the amount of solution was measured which was retainedwhen a pressure of 1 psi (4.5 kg/64 cm²) was applied for 15 minutes on afoam containing the above determined amount of solution and iscalculated according to the following formula: ##EQU1## wherein A_(m) isthe maximal amount of solution which the foam can absorb and A_(p) isthe amount of solution which remains in the foam after pressure wasapplied. The pressure was applied after allowing the foam to stand for1, 10, 20 or 30 minutes. In Table 5 the retention figures are given forthe optimal standing time(s).

The maximal absorption was determined by drying the foam for 24 h atroom temperature, completely immersing the foam in the solution for 15seconds, followed by taking the foam out of the solution and bydetermining the weight difference per dm³ of the foam after and beforeimmersing.

The above experiments were repeated using water at 22° C. The resultsare given in Table 5, experiments 11+12 (standing was 30 minutes).

                  TABLE 5                                                         ______________________________________                                                                      Retention,                                                                    %/optimal                                                           Maximal   standing                                                                              Core                                            Type of SAP/                                                                              absorption,                                                                             time(s),                                                                              density,                                Experiment                                                                            Amount of SAP                                                                             g/dm.sup.3                                                                              min     kg/m.sup.3                              ______________________________________                                        1       --          1370       53/1-30                                                                              65                                      2       1/15        1470       61/1-10                                                                              96                                      3       1/30        1350       57/1-10                                                                              99                                      4       1/50        1280      79/20   133                                     5       2/15        1240      67/10   90                                      6       2/30        1410      95/10   105                                     7       2/50         990      98/30   102                                     8       3/15        1480      64/10   80                                      9       3/30        1330      71/10   102                                     10      3/50        1250      99/10   128                                     11      1/50        1460      97      154                                             (T water =                                                                    22° C.)                                                        12      2/30        1360      94      124                                             (T water =                                                                    22° C.)                                                        ______________________________________                                         SAP Type                                                                      1: Polyacrylamidebased SAP; Molecular weight of about 5.10.sup.6.             2: Starchgrafted sodium polyacrylate SANWET IM 7000.                          3: Sodium polyacrylate SANWET IM 3900 G.                                 

EXAMPLE 4

Example 3, experiment 4 was repeated with the proviso that the SAP wasadded in different ways. The cells of the foams obtained were visuallyinspected as to whether or not open celled foam was obtained andshrinkage of the foam was measured according to the formula: ##EQU2##wherein S_(b) is the diameter of the cup wherein the foam was made andS_(a) is the diameter of the foam obtained after standing at roomtemperature for 12 hours.

The results are in Table 6, experiments 1-5.

                  TABLE 6                                                         ______________________________________                                        Experiment                                                                            Addition of SAP   open-cells                                                                              shrinkage                                 ______________________________________                                        1       to water, immediately                                                                           no        18                                                followed by prepolymer                                                2       to water, followed by                                                                           no        65                                                prepolymer addition after                                                     150 seconds and add                                                           prepolymer                                                            3       to prepolymer, immediately                                                                      yes       14                                                followed by water                                                     4       to prepolymer, followed by                                                                      yes       13                                                water addition after 150                                                      seconds                                                               5       SAP, prepolymer and water                                                                       yes       13                                                are combined together                                                 6 + 7   to prepolymer, followed by                                                                      yes       13                                                water addition after 2 hours                                          ______________________________________                                    

Experiment 4 was repeated with the proviso that instead of after 150seconds water was added after 2 hours and that SANWET IM 7000 and SANWETIM 3900 G were used in an amount of 30 pbw. The resuls are in Table 6,experiments 6+7.

EXAMPLE 5 (COMPARATIVE)

Example 3, experiment 3 was repeated with the proviso that 5 pbw ofwater was used. The foam collapsed.

We claim:
 1. Process for preparing a flexible foam by reacting aprepolymer having an NCO value of 3-15% by weight, which is the reactionproduct obtained by reacting an excessive amount of a polyisocyanatewith a polyether polyol or a mixture of such polyols, said polyol ormixture having an average nominal hydroxyl functionality of from 2 to 6,an average hydroxyl equivalent weight of from 500 to 5000 and anoxyethylene content of at least 50% by weight, with water, the amount ofwater being 15-500 parts by weight per 100 parts by weight ofprepolymer, at the start of the reaction the temperature of theprepolymer being 10°-50° C. and the temperature of the water being10°-50° C. higher than the temperature of the prepolymer.
 2. Process forpreparing a flexible foam according to claim 1, wherein at the start ofthe reaction the temperature of the water is 25°-90° C.
 3. Processaccording to claim 1 wherein the temperature of the prepolymer is15°-30° C., the temperature of the water is 40°-70° C. and thetemperature of the water is 20°-45° C. higher than the temperature ofthe prepolymer.
 4. Process according to claim 1 wherein the prepolymeris a prepolymer having an NCO value of 3-10% by weight which is thereaction product obtained by reacting an excessive amount of apolyisocyanate containing at least 65% by weight of 4,4'-diphenylmethanediisocyanate or a variant thereof and the polyol or mixture of polyolshas an average nominal hydroxyl functionality of from 2.5 to 3.5, anaverage hydroxyl equivalent weight of from 1000 to 3000, and anoxyethylene content of from 50 to 85% by weight and the amount of wateris 30-300 parts by weight per 100 parts by weight of prepolymer.
 5. Aprocess according to claim 1 wherein the reaction between the prepolymerand the water is conducted in the presence of 0.01-10 parts by weight,per 100 parts by weight of prepolymer, of a polyol (2) having an averagemolecular weight of 500-10000 and an average nominal hydroxylfunctionality of 2-6, this polyol being a polyoxyethylene polymer or apolyoxyethylene polyoxypropylene block copolymer having an oxyethylenecontent of at least 30% by weight.
 6. A process according to claim 5wherein polyol (2) has an average nominal hydroxyl functionality of 2and an oxyethylene content of 35-70% by weight.
 7. A process accordingto claim 5 wherein polyol (2) has an oxyethylene content of at least 70%by weight and the amount of water used is at least 40 parts by weightper 100 parts by weight of prepolymer.
 8. A process according to claim 1wherein the reaction between the prepolymer and the water is carried outin the presence of a superabsorbent polymer.
 9. A process according toclaim 8 wherein the superabsorbent polymer is used in an amount of 10 to70 parts by weight per 100 parts by weight of prepolymer.
 10. A processaccording to claim 8 wherein the superabsorbent polymer is selected fromsuper absorbent polymers based on acrylic or methacrylic acids, esters,nitriles, amides and/or salts thereof; polysaccharide basedsuperabsorbent polymers and superabsorbent polymers based on maleicanhydride.
 11. Process for preparing a flexible foam by reacting aprepolymer having an NCO value of 3-15% by weight, which is the reactionproduct obtained by reacting an excessive amount of a polyisocyanatewith a polyether polyol or a mixture of such polyols, said polyol ormixture having an average nominal hydroxyl functionality of from 2 to 6,an average hydroxyl equivalent weight of from 500 to 5000 and anoxyethylene content of at least 50% by weight, with water, the amount ofwater being 15-500 parts by weight per 100 parts by weight ofprepolymer, characterised in that the reaction of the prepolymer and thewater is conducted in the presence of a superabsorbent polymer. 12.Process according to claim 11 wherein the prepolymer is a prepolymerhaving an NCO value of 3-10% by weight which is the reaction productobtained by reacting an excessive amount of a polyisocyanate containingat least 65% by weight of 4,4'-diphenyl methane diisocyanate or avariant thereof and the polyol or mixture of polyols has an averagenominal hydroxyl functionality of from 2.5 to 3.5, an average hydroxylequivalent weight of from 1000 to 3000, and an oxyethylene content offrom 50 to 85% by weight and the amount of water is 30-300 parts byweight per 100 parts by weight of prepolymer.
 13. A process according toclaim 11 wherein the reaction between the prepolymer and the water isconducted in the presence of 0.01-10 parts by weight, per 100 parts byweight of prepolymer, of a polyol (2) having a number average molecularweight of 500-10000 and an average nominal hydroxyl functionality of2-6, this polyol being a polyoxyethylene polymer or a polyoxyethylenepolyoxypropylene block copolymer having an oxyethylene content of atleast 30% by weight.
 14. A process according to claim 13 wherein theamount of polyol (2) is 0.05-3 parts by weight per 100 parts by weightof prepolymer.
 15. A process according to claim 13 wherein polyol (2)has a nominal hydroxyl functionality of 2 and an oxyethylene content of35-70% by weight.
 16. A process according to claim 13 wherein polyol (2)has an oxyethylene content of at least 70% by weight and the amount ofwater used is at least 40 parts by weight per 100 parts by weight ofprepolymer.
 17. A process according to claim 11 wherein thesuperabsorbent polymer is used in an amount of 10 to 70 parts by weightper 100 parts by weight of prepolymer.
 18. A process according to claim11 wherein the superabsorbent polymer is selected from superabsorbentpolymers based on acrylic or methacrylic acids, esters, nitriles, amidesand/or salts thereof; polysaccharide based superabsorbent polymers andsuperabsorbent polymers based on maleic anhydride.